To fully understand the usefulness of the invention as a teaching tool, a brief review of the structure and function of DNA is required. DNA is the basic code for creating most living things on this planet. As a result of its power and usefulness, the importance of teaching and explaining the structure and function of DNA has become a high priority.
The structure of DNA was ascertained by Watson and Crick, and in 1953, a biology textbook was able to state, "[t]he age old question of how hereditary information is duplicated and passed on, duplicated and passed on, for generation after generation had in principle been answered." Watson and Crick were able to determine that the basic shape of the molecule was a double helix. This particular double helix has four nucleotide bases, namely, Adenine, Thymine, Cytosine, and Guanine. These four nucleotide bases are generally abbreviated A, T, C, and G.
The DNA molecule (FIG. 1) is made-up of two strands, aligned parallel to each other. The strands are formed by the bonding of the sugar (deoxyribose) component and the phosphate component of the DNA molecule. These strands make-up the double helix which usually coils clockwise. Perpendicular to the strands, and bonded to the sugar components, are the bases. The strands are joined together by the pairing of the bases, which bond together between the two helical strands. There are two classifications of bases, Purines and Pyrimidines. The Purine molecules are larger than the Pyrimidines molecules. Adenine and Guanine are Purines and Cytosine and Thymine are Pyrimidines.
The sequence of the nucleotide bases A, G, C, and T along the helical strand is all important because it conveys the hereditary information contained in the DNA molecule. This information depends on the order in which the base pairs are arranged along the helical strand. There are no limitations upon this order, the bases can be arranged in absolutely any sequence. There is, however, a stringent base-pairing rule: T can pair only with A, and C can pair only with G, more generally a Purine can only pair with a Pyrimidine. This is more clearly apparent when the difference in size of the Purine and Pyrimidine molecules are taken into account. In order to maintain the structure of the strands of the double helix, the strands must be substantially equidistant from each other. For this to occur the Purine must only bond to a Pyrimidine due to the difference in size between the molecules, hence the base pair rule. The DNA molecule thus consists of a chain of A-T and C-G base pairs which can be put together in any order up to hundreds of millions in a single chain.
The strands of the DNA molecule are made-up of two molecules, the sugar and the phosphate molecules. Each sugar has two different sites at which a phosphate molecule can bind, the 3' (three prime) site and the 5' (five prime) site. As a result each strand is formed by . . . -phosphate-sugar-phosphate-sugar- . . . bonds. It is important to note that each strand has directionality, meaning that one strand has an order of 5'-3'-5'-3'-5'-3' and as a result the opposing strand will have an order of 3'-5'-3'-5'-3'-5'.
When DNA duplicates itself it is called replication; replication occurs just prior to cellular division. In order for the DNA molecule to replicate, it must first unwind from the double helix into a form called the "ladder", and second, the A-T and C-G bonds must be broken and the molecule unzips to allow the two strands to separate. When the molecule is unzipped the bases on both strands are exposed to allow free-floating nucleotides to bond with them thereby creating a copy of both of the DNA strands.
Another important function of DNA is that it is the code for the making of proteins. The first step in making a protein is called transcription. Generally, transcription follows the same process as replication, however, the main difference is that DNA is copied into RNA instead of DNA. RNA has a very similar structure to DNA with a few differences, principally, RNA is single stranded and has four nucleotide bases with one significant difference. The four nucleotide bases that make-up RNA are Adenine, Cytosine, Guanine, and Uracil. Uracil takes the place of Thymine in DNA. RNA adheres to the following base pair rules: A binds only to U, and C binds only to G. Each combination of three bases code for a particular amino acid. Each combination of three bases is called a codon. There are many combinations of three bases which can be made (e.g., ATC, GCA, TCA). There are usually twenty amino acids used by mammals. Some codons code for the same amino acid. Amino acids are the building blocks for proteins, which serve a multitude of critical functions in living organisms. Because a particular codon codes for a particular amino acid, it is important in which direction the combination of three bases is read, for example, CAT does not code for the same amino acid as does TAC. Therefore, in order for DNA to function properly, the molecule must have some type of directional indicators. The model incorporates such directionality.
Currently, models of the DNA molecule are being used to teach and inform people about its structure. There is, however, at least one significant deficiency associated with these models, they are static. As a result of being static these models cannot teach people how DNA functions, this is left to drawings and discussion. How DNA functions is a very important concept and often difficult to understand, as such the art is amenable to useful refinement.